repository: fix pull when fetch returns up-to-date

[mcuadros]

After perform a Repository.Fetch if a Repository.Pull was done, the references weren't updated.

The reason was because Repository.Fetch returns NoErrAlreadyUpToDate and the internal Repository.updateReferences was not called

Fixes: #198

[smola]

This does not seem the standard git behavior:

➜  go-git.v4 git:(master) ✗ git fetch origin
Enter passphrase for key '/home/smola/.ssh/id_rsa': 
remote: Counting objects: 18, done.
remote: Compressing objects: 100% (18/18), done.
remote: Total 18 (delta 4), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (18/18), done.
From github.com:src-d/go-git
   92d3de7..c5f1056  master     -> origin/master

➜  go-git.v4 git:(master) ✗ git pull origin master
Enter passphrase for key '/home/smola/.ssh/id_rsa': 
From github.com:src-d/go-git
 * branch            master     -> FETCH_HEAD
Updating c9c043c..c5f1056
[...]

➜  go-git.v4 git:(master) ✗ git pull origin master
Enter passphrase for key '/home/smola/.ssh/id_rsa': 
From github.com:src-d/go-git
 * branch            master     -> FETCH_HEAD
Already up-to-date.

First I fetch, then pull, and pull again. First pull does not return "Already up-to-date" because it updates a reference, second pull returns "Already up-to-date" because it doesn't.

[codecov-io]

Current coverage is 76.26% (diff: 76.31%)

Merging #207 into master will decrease coverage by 0.57%

@@             master       #207   diff @@
==========================================
  Files            96         96          
  Lines          6278       6298    +20   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits           4824       4803    -21   
- Misses          925        973    +48   
+ Partials        529        522     -7   

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